The present invention relates to methods of identifying drug-resistant and/or drug-sensitive cells, for example, breast cancer and brain tumor cells, on the basis of different ion and/or second messenger dynamics between a drug-sensitive and drug-resistant cell. For example, the invention provides measuring the comparative decay rates of a cellular ion, such as calcium, released into the intracellular compartment of drug sensitive and/or drug resistant cells. The present invention also provides methods for screening compounds that modulate the ionic dynamics of a cell as well as methods of determining drug resistance/sensitivity of cancer cells from cancer patients and/or designing cancer therapy based on of the ionic dynamics of cancer cells from a particular patient.